The stock player won't play 24/96 FLAC files if those are what you mean by hi def. You get the message that's unsupported. It does play 16/44.1 FLAC so it doesn't appear that the codec is the problem, it's the bit depth or sample rate.

Other 3rd party player apps I tried will play them but i don't know if they're being downsampled during playback. Except for Rockbox. That resamples everything that 16/44.1 using linear interpolation. I'd think you would need to contact the developers of the other apps to find out what they do.

I've heard of some 3rd party apps that advertise audiophile playback so I'd guess they do if they can. I never followed up on any of that because I don't use any files at anything else other than 16/44.1. I'm not taking up space on a portable player storing bits I can't hear.